The present invention relates a chuck for holding a workpiece by a wedge action on a machine tool and more particularly a precision chuck capable of holding a workpiece by a stronger holding force with a high degree of accuracy.
A chuck of the type described above is, for example, disclosed in Japanese Patent Publication No. 57-46965 and is shown in FIGS. 1 and 2. In this chuck, an axially movable annular plunger 3 is fitted into a central bore 2 formed through a body 1 and is formed with a plurality of equiangularly spaced apart T-shaped wedge grooves 4. The body 1 is formed with a plurality of axially extended guide grooves 5 into which are axially slidably fitted master jaw 6 connected top jaw adapted to chuck a workpiece. The rear inner end portion remote from the corresponding top jaw of the master jaw 6 is formed with a T-shaped wedge portion 7 adapted to fit into the corresponding T-shaped wedge groove 4 of the plunger 3. The front surface of the master jaw 6 is formed with front guided surfaces 8 while the rear surface thereof is formed with rear guided surfaces 9 and these front and rear guided surfaces 8 and 9 are in opposed relationship with side surfaces 10 of the guide groove 5 of the body 1. A cover 11 for preventing the intrusion of chips and dust particles is fitted into the plunger 3.
The chuck with the above-described construction has a defect that the length L of the contact between the side surface 10 of the guide groove 5 of the body 1 and the front guided surface 8 of the master jaw 6 is greater than the length 1 of the contact between the side surface 10 of the guide groove 5 of the body 1 and the rear guided surface 9 of the master jaw 6 that is, L&gt;1.
Because of this difference in the length of contact, there exists a great difference in the displacement or floating of the master jaw 6 under an external force between the time when the external peripheral surface of a workpiece is held and the time when the internal peripheral surface of a workpiece is held so that it results in a difference in workpiece holding accuracy between the case in which the inner peripheral surface of a workpiece is held and the case in which the outer peripheral surface of a workpiece is held. In addition, the friction between the rear guided surface 9 of the master jaw 6 with the body is greater than the friction between the front guided surface 8 of the master jaw 6 with the body, the rear guided surface 9 is quickly worn out.
As a result, the workpiece-holding-accuracy in the case of holding the inner peripheral surface of a workpiece 13 becomes lower than in the case of holding the outer peripheral surface of a workpiece 13 and moreover there arises the problem that the sliding resistance encountered by the master jaw 6 in case of holding the inner peripheral surface of a workpiece becomes higher than that in case of holding the outer peripheral surface of a workpiece.
In order to eliminate the above and other defects encountered in the prior art chucks, Japanese Patent Application Laid-Open No. 60-94207 discloses a chuck as shown in FIGS. 3 and 4. The chuck has a two-split body 20 consisting of an axially front body section 21A and an axially rear body section 21B and a center hole 22 is bored through the front and rear body sections 21A and 21B. The front body section 21A is provided with a plurality of axially extended guide grooves or recesses 23 and a master jaw 24 is slidably fitted into respective guide grooves or recesses 23 for slidable movement in the axial direction of the front body section 21A. An annular cylinder 25 is interposed between the front and rear body sections 21A and 21B and an annular piston 26 is fitted into the cylinder 25 for slidable movement in the axial direction. One end of the piston 26 is made into threadable engagement with a base portion 28 of a plurality of wedges 27 which are in opposed relationship with the master jaw 24. Each wedge 27 is provided with a pair of wedge main bodies 29 which are extended from the base portion 28 and which are spaced apart from each other by a predetermined distance. Each wedge main body 29 is so tapered that its height (or thickness) is gradually increased toward its front end. Meanwhile each master jaw 24 is formed with tapered guide surfaces 30 on both sides of the radial centerline thereof for engagement with the wedge main bodies 29 extended from the base portion 28.
With the above-described construction, the wedge main bodies 29 of the wedge 27 are made into engagement with each other at the radial centerline of the master jaw 24 so that the workpiece-holding-accuracy can be well balanced as compared with the prior art chuck in which the wedge main bodies 29 are made into engagement with each other at the radially inward portion and at the rear portion in the axial direction of the master jaw 6.
However, the body 20 consists of the front and rear body sections 21A and 21B split in the axial direction so that the guide surface 30 of the front body section 21A which supports the wedge main body 29 of the wedge 27 and the inner peripheral surface of the cylinder 25 of the rear body section 21B which supports the piston 26 threaded into the base portion 28 of the wedge 27 must be machined by completely different machining means and steps. As a result, there arises the problem that it becomes impossible to maintain the guide surface 30 in completely coplanar relationship with the inner peripheral surface of the cylinder 25 in the axial direction.
Because of the above-described problem, only the inner peripheral surface of the cylinder 25 which guides the outer peripheral surface of the sliding piston 26 becomes a reference surface for guiding the wedge 27. As a result, the guide surface 30 cannot support the wedge main bodies 29 in a stable manner so that there is a defect that the base portion of the wedge main body 29 of the wedge 27 is cracked. More particularly, when the driving force exerted to the piston 26 is transmitted through the wedge 27 to the master jaw 24, the relative slidable movement between the inner peripheral surface of the cylinder 25 and the outer peripheral surface of the piston 26 with a minimum gap therebetween is maintained, thereby maintaining a desired degree of workpiece-holding holding-accuracy. Furthermore, the wedge main bodies 29 are merely fitted into axial holes 31 of the front body section 21A and the wedge main bodies 29 of the wedge 27 are made into engagement with the axial holes 31 of the front body section 21A only under the following conditions. In order to maintain a satisfactory degree of workpiece-holding-accuracy in the conventional machining processes, the wedge main bodies 29 are so designed and constructed to have a sufficient degree of rigidy so that they may be used as a cantilever. That is, when the machining force which is externally exerted to a workpiece increases beyond a predetermined point so that excessive loads are exerted to the wedge main bodies 29, the axial holes 31 then support the wedge main bodies 29 to bear part of the excessive loads exerted thereto so that the base portion of the wedge main bodies 29 is prevented from being cracked. In this case, the base portion of the wedge main bodies 29 is considerably deformed so that the workpiece-holding-accuracy is naturally degraded.
Therefore, the chuck disclosed in Japanese Patent Application Laid-Open No. 60-94207 and described above with reference to FIGS. 3 and 4 cannot attain a sufficient holding force and is not adapted to be rotated at high rotational velocities. As a result, it cannot be used in the recently-developed high-rotational-velocity machine tools.